1. Field of the Invention
The present invention relates to fabricating integrated circuits, and more specifically, to a method for fabricating a metal-oxide-semiconductor (MOS) transistor.
2. Discussion of the Related Art
FIGS. 1 through 4 illustrate a sequence of steps in a CMOS process. In particular, FIG. 1 shows an epitaxial region 106 on semiconductor substrate 100. Semiconductor substrate 100 can be, for example, a p-type monocrystalline semiconductor, with a thin (5-10 .mu.m thick), lightly doped P-type epitaxial layer 106 at the surface
An N-well region (e.g., N-well region 104) is typically provided in epitaxial layer 106 in an "active" region (i.e., where a transistor can be fabricated). To form N-well region 104, a thin oxide 102 (e.g., typically 100-500 .ANG. thick) is grown on epitaxial layer 106. A silicon nitride (SiN) layer 108 is then deposited, patterned and etched. Then, as shown in FIG. 2, a LOCalized Oxidation of Silicon (LOCOS) process is carried out. The exposed portions of the semiconductor surface (i.e., those portions not protected by silicon nitride layer 108) are oxidized to form a thick thermal oxide ("field oxide") 114. The portions of semiconductor surface with thermal oxide 114 is referred to as the field regions. The portions of the semiconductor surface protected by silicon nitride layer 108 become the active regions, where active semiconductor devices are to be fabricated. Thermal oxide 114 isolates devices in active regions from each other. Silicon nitride layer 108 is then removed.
After a masking step creating mask 112, an ion implantation step 116 is carried out through mask 112 to form N-well region 104. Phosphorus or arsenic can be used to form an N-well while boron or BF.sub.2.sup.+ can be used to form a P-well. The impurities are then driven in to the appropriate depth during subsequent high temperature cycles. At the conclusion of the drive-in processes the surface concentration in N-well region 104 can be .about.1.times.10.sup.16 /cm.sup.3 and the surface concentration in a typical P-well is .ltoreq.1.times.10.sup.15 /cm.sup.3. The mask is then removed from the active regions, as shown in FIG. 3 (prior art). NMOS devices can be formed in epitaxial layer 106 or in P-wells, while the PMOS devices are formed in doped (e.g., 1.times.10.sup.16 /cm.sup.3) N-well region 104. A threshold adjust implant can then be carried out, followed by growing of a gate oxide.
Polysilicon is deposited by CVD as a material for forming gate electrodes. The polysilicon layer is subsequently doped to form a polysilicon gate material. Polysilicon can be doped either by diffusion or by ion implantation after deposition, or in situ during deposition. The polysilicon is then patterned to form polysilicon gates, such as those designated by reference number 116 in FIG. 4 (prior art).
Next, polysilicon gates 116 are used as a mask to selectively implant the source/drain regions 120 of the transistors. Each of polysilicon gates 116 protects the channel region under the gate from being implanted by dopants 118. Arsenic is the preferable dopant for the n+ source/drain regions (NMOS devices), so that shallow junctions and minimum lateral diffusion under the gate can be obtained. Arsenic is typically implanted to a dose of 3-6.times.10.sup.15 cm.sup.-2, and with an energy of 40-60 keV. Boron is often implanted as BF.sub.2 + for the p+ region (for shallow junction formation) at doses of 1-5.times.10.sup.15 cm.sup.-2 and energies of 30-50 kev. These implants are usually performed through a screen oxide to protect source/drain regions 120 from contamination during the implant procedure. These implants are then annealed with a short thermal process at a temperature ranging from 800-1100.degree. C.
In the above-described conventional CMOS process, the implant area is generally defined by an oxide or other masking material opening on the surface of the silicon. The size of this opening is determined by the resolution of the photoresist process which can be limited by many aspects of the process, including hardware (e.g. diffraction of light, lens aberrations, and mechanical stability of the system); optical properties of the resist material (e.g. contrast, swelling behavior, and thermal flow); and process characteristics (e.g. softbake step, develop step, postbake step, and etching step). Thus, the size for the diffusion regions which are the source and the drain of the transistor are limited by the photoresist process.
A process for fabricating an MOS transistor having a narrow diffusion region smaller than the resolution of a photo resist process, using conventional CMOS fabrication techniques, is desired.